This invention relates to a method of controlling fires and more particularly to a method of controlling toxic outgassing of certain burning or smouldering materials.
In recent years polymeric materials have been used more and more for structural components and for interior furnishings of buildings. Many of these materials contain halogen, cyanide, and/or other nitrogenous mixtures, which can react during combustion or pyrolysis to form toxic products such as hydrogen chloride (HCl) and hydrogen cyanide (HCN). Because of the danger to life and property, the response of these materials to fire needs to be readily controlled.
Experience has shown that one must be careful in interpreting the terminology used to describe varying degrees of fire resistance. Self-extinguishing materials burn as long as a source of external heat is present but are incapable of sustaining combustion after removal of the heat source. The fire characteristics of a self-extinguishing grade of polyurethane foam can be drastically changed by the processing it undergoes to produce a good microwave absorber. Treatment with fire retardants does not produce a nonflammable material; it only reduces flammability. A nonflammable material will not burn when exposed to a flame; however, foams that have been painted with a fire-retardant paint can lose their fire-retardant characteristics when the paint is destroyed, either mechanically or by fire.
The interior walls and ceiling of many modern anechoic chambers are covered with a nitrogen-containing polymer-polyurethane foam. To serve as a good microwave absorber, the foam is impregnated with a carbon and a latex binder to impart good electrical characteristics. The resulting material has low electrical resistance and is thus a good conductor. The foam is further treated with enough of a fire retardant, such as a chlorine-containing compound, to impart fire resistance with degrading electrical properties. When involved in fires, these treated foams may release toxic combustion products containing nitrogen and halogen, whose presence could significantly increase the toxicity of fire gases.
Further, it has been determined that these polymers release the irritant gas, hydrogen chloride (HCl). When air containing moderate concentrations of HCl gas is inhaled, the HCl is normally removed by the upper respiratory tract and does not penetrate into the lungs. However, it has been shown that the gas may be sorbed in hazardous quantities on airborne, fire-generated particles. If the particles are properly sized when inhaled, they carry the irritant gas past the defenses of the upper respiratory tract and deposit in the lungs. Desorbing from the particles, the gas then insults the delicate tissue of the lungs and causes pulmonary edema. This mechanism is believed to have caused deaths that occured 24 to 48 hours after exposure in certain fire situations.
Airborne soot and water particles are the suspected carriers of the irritant gas in fires. In burning experiments of polyvinyl chloride with polyethylene, soot particles have been shown to carry the irritant. When filtered from the fire gases, 2 percent by weight of the particles have been found to be loosely bound HCl. Electron micrographs show that the particles are sperically shaped with diameters of from 0.03 to 0.11 microns. According to theory, such particles cluster into assemblies that, for periods of 30 to 60 minutes, are of sizes that penetrate the respiratory defenses; 20 to 40 percent of the inhaled particles succeed.
Atmospheric analysis of different types of foams indicate that the combustion products contain toluene diisocyanate, HCN, HCl, CO.sub.2 and CO. These gases will cause discomfort in a short period of time and even death after an hour in some cases. Experimental tests have been reported in an NRL Report 7793 entitled "Flammability and Toxic-Gas Production from Urethane Foams used in Anechoic Chambers", by Patricia A. Tatem and Frederick W. Williams, Naval Research Laboratory, Washington, D.C. 20375.
The almost opaque airborne soot compounds the fire fighter's problem. Visibility is so poor that personnel must be continually accounted for and the buddy system used.
The problem with burning foam is it will burn as long as heat is applied, even if it has a fire retardant in the foam. There is little firemen can do to control smouldering RF absorber material other than contain products of combustion and wait for smouldering to gradually stop. Normal use of water, foam, CO.sub.2 and soda acid extinguishers do nothing to control outgassing. In most RF absorbers, even fire retardant polyurethane, smouldering continues unless the temperature is lowered because the material contains its own oxidizer and can burn slowly for hours. Therefore, some method must be used to extinguish the burning and smoulding foam materials while preventing toxic outgassing.